Power measurement devices for bicycles typically fall into one of two basic categories: devices that measure torque and angular velocity in driving components of a bicycle, such as pedals and crank arms, and devices that measure torque and angular velocity in driven components such as the chain rings, chain and rear wheel. Power P is the product of torque T and angular velocity xcfx89, that is, P=Txc3x97xcfx89. Similarly, power P can be defined as the product of force F and velocity v, i.e., P=Fxc3x97v.
Certain prior devices that measure torque in driving components have inherent inaccuracies with regard to measuring either the true power exerted by a bicyclist or the true power transmitted to the driven wheel of a bicycle. The total force exerted by a bicyclist will include forces that are applied tangentially to a driving component, e.g., crank arm, and forces that are applied either in another direction in the plane of rotation or in a plane that is perpendicular to the plane of rotation. The portion of force that is tangential to the direction of rotation, when multiplied by the distance from the center of rotation to yield torque and then multiplied by angular velocity, represents a measure of power transmitted to a driven wheel. However, the forces that are in another direction in the plane of rotation and the forces that are perpendicular to the plane of rotation cannot be added to the forces causing rotation, multiplied by angular velocity, and represented as a true measure of power transmitted to the driven wheel. Nor can the total force be multiplied by the angular velocity of a driven component to represent the power exerted by the bicyclist.
U.S. Pat. No. 4,141,248 to Bargenda discloses a torque measurement means which includes a wire strain gauge operatively connected to each of the foot pedal armatures of a bicycle and connected in series in a strain gauge bridge. This means of measuring torque has limited accuracy since it is known that the force exerted into the foot pedals and resolved by the instrumented aramtures is not limited purely to the measurement of force that will be transmitted to the driven wheel. The instrumented armatures will also be affected by significant forces that are perpendicular to the desired torque measurement thereby diluting the significance of the measured power that is assumed to be transmitted back to the driven wheel. Although this means provides a measurement of the exerted forces produced by a cyclist, it is not an accurate measurement of force and power that is usefully produced and efficiently transmitted to the driven wheel.
U.S. Pat. No. 4,423,630 to Morrison discloses a means of measuring power by measuring the force in a driving foot pedal of a bicycle. This method of force measurement also exhibits inaccuracies in that a significant proportion of force registered by the pedals is not transmitted via the bicycle chain.
In general, devices that measure torque in driven components have the potential of registering a more accurate measure of torque and power usefully transmitted by the cyclist to the bicycle wheel. It should be noted that this is not the same as the power exerted by the cyclist. Some of these prior devices include methods of detecting torque in the hub of the driven wheel. The torque in the hub is usually detected by some indirect means such as by optically detecting relative deflection between components in the wheel and converting the detected deflection to a torque measurement.
U. S. Pat. No. 4,811,612 to Mercat discloses a method and device for detecting torque in the rear wheel of a bicycle. Under applied torque to the wheel, special radial spokes deform such that the rim of the wheel angularly deflects relative to the hub of the wheel. Using pairs of infrared-radiation-emitting diodes and photo diodes mounted adjacent to the wheel hub and rim, the device optically detects the angular position of the rim compared to that of the hub by measuring delay in the signals generated by the photo diodes.
U.S. Pat. No. 4,966,380 to Mercat discloses a device used to detect torque in the hub of the rear wheel of a bicycle. The device includes a pair of mating rotatable members which rotate about the axle of the bicycle. The mating members include spiral ramps in contact with each other such that as torque is applied to the hub, one of the mating members is displaced axially along the rotational axis of the hub against a stationary flexible disk. A strain gauge on the stationary flexible disk detects the deflection of the disk. A processor converts the stationary disk deflection measurement into a torque measurement. The device of the ""380 patent addresses a problem seen by Mercat as being associated with applying a sensor to the rotating portion of the hub. Mercat therefore uses a stationary detection member, not in the rotational portion or torque-carrying path of the hub, to detect torque in the hub.
U. S. Pat. No. 5,065,633 to Mercat discloses another device used to detect torque in a bicycle hub. The device includes an outer hub and an inner driving axle which are rotatably mounted over the stationary axle of the bicycle and are fixedly attached to each other at a fixed end of the hub and are allowed to rotate relative to each other under applied torque at the opposite free end of the hub. Torque applied to the driving axle is coupled to the outer hub at the fixed end of the hub to rotate the bicycle wheel.
In the ""633 patent, the torque measuring device is coupled off of the torque path to the torque-carrying members, i.e., the driving axle and outer hub, to detect relative deflection between the outer hub and the stationary axle at the free end of the hub. A multiplying arm is pivotally attached at one end to the outer hub and rests against a rotatable carrier element at its other end. Magnets are attached to the carrier element at the free end of the hub and the fixed end of the hub and are located in registration with mating magnetic reed contacts mounted on the stationary axle. When torque is applied to the driving axle, the hub rotates slightly with respect to the driving axle. The multiplying arm pivots to move the carrier element and the magnets on the carrier element at the free end of the hub relative to the magnets at the fixed end of the hub. Signals generated by the stationary reed contacts are analyzed to relate the amount of deflection in the outer hub to applied torque.
U. S. Pat. No. 5,031,455 discloses another device used to detect torque in a bicycle hub. The device includes an inner hub and an outer hub connected by torsion bars. The inner hub is driven into rotation and carries the outer hub and the bicycle wheel into rotation via the torsion bars. A pair of discs mounted to the inner hub and outer hub are used to detect relative rotation between the inner and outer hubs. The discs include a plurality of circumferential openings which are aligned under zero torque but are displaced relative to each other under applied torque. The device uses optical or magnetic sensing to detect the amount of relative displacement, which is in turn related to the applied torque.
All of these devices use indirect means to measure torque. Each detection approach carries out torque detection in elements which are outside the path along which torque is coupled through the bicycle driving system. As a result, certain inaccuracies are inherent to such systems, such as those introduced by losses in coupling the special torque detecting components to the actual wheel driving components. Also, the indirect detection approaches used, i.e., optical, magnetic, can also introduce inaccuracies. These approaches rely heavily on the accuracy of the associated components, e.g., the discs and the openings formed therein. Any inaccuracies in fabrication or assembly of any of these associated components can adversely affect the accuracy of the torque measurement.
Also, these approaches generally require large angular deflections to achieve an accurate measurement of torque in the hub of the driven wheel. These large angular deflections are unacceptable for performance bicyclists. For example, even a full-scale deflection of an ostensibly small angle of 1 degree would translate to a 0.25 inch deflection at the outside of a 27 inch bicycle wheel. Such a deflection would be annoyingly noticeable to a performance cyclist.
The present invention is directed to an apparatus and method for measuring torque and/or power in a driven wheel, for example, a bicycle wheel. The apparatus of the invention includes a torque coupling member which is mountable within the driven wheel and which couples a rotational driving force through the driven wheel to drive the driven wheel into rotation, that is, the torque coupling member is in the path along which torque is coupled through the wheel. The apparatus also includes a sensing means located on the torque coupling member for generating a signal indicative of strain in the torque coupling member.
In one embodiment, the torque coupling member is mountable in the hub assembly of the wheel. In one particular embodiment, the torque coupling member forms a portion of the hub assembly which couples the torque through the hub assembly to drive the wheel into rotation and rotates with the hub assembly and wheel as the wheel rotates. Hence, in this embodiment, the sensing means generates the signal indicative of strain in the torque coupling member as the torque coupling member rotates in response to the applied torque.
In one embodiment, the hub assembly in accordance with the invention includes a first or outer hub member and a second or inner hub member within the outer hub member. The inner hub member is coupled to a wheel driving member such as a bicycle chain sprocket to couple rotational driving forces into the wheel. The outer hub member is coupled to the outer portion of the wheel such as the bicycle wheel spokes or solid disk. The inner hub member is coupled to the outer hub member such that as the inner hub member is driven by the rotational driving forces, the outer hub member and, therefore, the wheel, are driven into rotation.
The hub members can be coupled by one of several different means. In one embodiment, a link member is attached to both hub members. The link member can be positioned at one or both ends of the combined inner and outer hub member subassembly, or at any position or positions along the length of the subassembly. Alternatively, the inner and outer hub members can be joined by a fastening member which can include one or more screws or pins. The fastening member can also include one or more keys in one of the hub members mating with one or more keyways in the other hub member. In another configuration, the fastening member can include a spline at one end of the inner and outer hub member subassembly. In yet another configuration, the inner and outer hub members are formed as a single unitary piece, such as by molding or machining.
In one embodiment, the sensing means is mounted on the link member that couples the inner and outer hub members together. The sensing means can include one or more strain gauges bonded to one or more bending beams of the link member. Tension and/or compression strain due to bending of the bending members can then be measured to determine the amount of torque applied to the hub. In one embodiment, two bending beams are used. In an alternative embodiment, four are used. Alternatively, the link member can include a shear web region which can be subject to shear and/or bending forces under applied torque. In this embodiment, one or more strain gauges can be bonded to the shear web region of the link member to measure tension and/or compression strain due to the shear and/or bending forces in the link member. These measurements can then be used to obtain a measurement of the applied torque. Bending beams can also be combined with the shear web region in the link member to measure both shear and bending strain.
In another embodiment, the sensing means is mounted on one of the inner and outer hub members. In this embodiment, one of the more rigid configurations for attaching the inner and outer hub members, for example, the screws, pins, keys, spline, or the unitary subassembly, can be used. The sensing means can include one or more strain gauges attached to the inner or outer hub member. In one specific embodiment, the strain gauges are attached to the inner hub member. They can be arranged to measure tension and/or compression strain due to torsional, shear and/or bending forces in the hub member such that torque applied to the hub member can be determined. By using a bearing on one or both ends of the hub assembly, the inner hub can be isolated from all external forces other than the driving torque it is meant to measure.
In accordance with the invention, the strain gauges can be mounted on the torque coupling member, e.g., the link member or the inner hub member, in any one or more of many possible configurations, depending upon the particular type of strain measurement desired. For example, in the case of bending beams on the link member, strain gauges can be oriented to measure tension and/or compression under the bending motion of the beams. In the case of the shear web region on the link member, strain gauges can be mounted to sense tension and/or compression in the shear web region due to shear and/or bending forces. In the case in which the strain gauges are mounted on the hub member, they can be mounted to detect tension and/or compression in the hub member due to torsional, bending and/or shear forces applied to the hub member.
The invention can also include a means for determining the angular velocity of the wheel as it rotates. The angular velocity measurement can be combined with the torque measurement to determine power delivered to the wheel. In one embodiment, the means for determining angular velocity includes a magnetic reed switch mounted on the hub assembly and a stationary magnet mountable in fixed relation to the bicycle axle. As the reed switch passes the magnet, the switch is activated to produce a signal such as a pulse. The signals are analyzed to determine angular velocity of the wheel. The angular velocity can then be used to calculate power delivered to the wheel, the velocity of the bicycle, the distance traveled by the bicycle and other desired parameters.
The electronics required to support the strain, torque, power and angular velocity measurements of the invention can be provided on the hub assembly in accordance with the invention. In one embodiment, an electronics unit is mounted on the hub assembly. The electronics unit can include a battery for powering the electronics and applying the required signal to the strain gauges. Processing, amplification and conversion circuitry and a transmitter, such as a radio frequency (RF) transmitter, can be provided for processing and transmitting strain and angular velocity measurement signals to a receiver, which can then further process the signals and transfer them to a computer. The computer can then provide numerous functions, such as computing torque and power using the strain and angular velocity signals and displaying results in predetermined formats. The electronics unit can include the required circuitry on one or more printed circuit boards and can include a housing to protect the circuitry from environmental hazards.
The apparatus and method of the invention provide numerous advantages over prior approaches. The invention provides more. precise and accurate torque and power measurements because it obtains direct strain measurements from the hub in the actual hub components that couple the applied torque through the wheel using stain sensing means that are inherently more precise and accurate. The inaccuracies introduced by the indirect measurement approaches of prior systems, using sensing means that are applied to torquing components indirectly and are therefore inherently less accurate and precise, are virtually eliminated.